When does hydroxide abstract an alpha H vs attacking a carbonyl carbon?

[d999]

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Maybe the better question is when does hydroxide act as a base vs. nucleophile? In a gem-diol rxn under basic conditions -OH acts as a :nu attacking carbonyl carbon. In an aldol condensation the -OH functions as a :nu and abstracts alpha H. I haven't had an aldol condensation question where I've also had the conditions (acidic vs basic). How do you predict how -OH behaves?

 

[Entadus]

I responded to your other thread.
http://forums.studentdoctor.net/thr...alpha-h-vs-attacking-carbonyl-carbon.1056914/